The long-term objective of this research is an understanding of the central neural circuits regulating body temperature and the mechanisms through which their function is altered during the fever response, an important component of the acute-phase reaction to immune and inflammatory stimuli. In uncontrolled conditions, fever can threaten cellular homeostasis and survival. Treating such dysregulation of body temperature would be aided by an understanding of the functional organization and neurotransmitters within the neural pathways that are activated by endogenous pyrogens. The proposed research plan is a logical extension of the results from the initial funding period to directly test a series of specific hypotheses represented in a model of the central neural circuits through which increased prostaglandin (PGE2) synthesis in the medial preoptic hypothalamus results in activation of brainstem sympathetic premotor neurons controlling brown adipose tissue (BAT) thermogenesis, cutaneous vasoconstriction and heart rate. These studies will provide, for the first time, the unique opportunity to define each of the synaptic integration sites in the pathways by which hypothalamic neurons regulating thermal homeostasis activate three important sympathetically-regulated targets to produce an increased body temperature during fever. The proposed studies to elucidate the organization, function and pharmacology of the longitudinally-organized, core pathway for the PGE2-mediated, sympathetic activation of BAT thermogenesis, cutaneous vasoconstriction and heart rate will combine data from anatomical tracing and immunocytochemical experiments with those from studies examining the effects on sympathetic outflows from microinjection of substances into specific brain regions to guide subsequent electrophysiological recordings from single neurons at sites in the hypothalamus, pons and medullary raphe pallidus. The three specific aims will focus on the relevant anatomical connections, the evoked physiological responses and the behavior of functionally-identified neurons in (1) the dorsomedial hypothalamus, (2) the ventromedial periaqueductal gray and (3) the medial preoptic hypothalamus, respectively, to test clearly defined hypotheses on their functional roles in the activation of RPa neurons that leads to increased heat production, heart rate and heat retention evoked during the febrile response to PGE2 in preoptic hypothalamus. Understanding the pharmacology within this pathway driving the sympathetic responses during fever could provide a foundation for developing strategies to alter sympathetically-mediated increases in body temperature.